The present invention relates to the design and construction of a reinforced matrix, typically a laminate, as well as to processes for manufacturing such laminates.
The present invention relates more particularly to a fiber reinforced plastic laminate substrate having one or more fiber free areas arranged in a pattern designed to facilitate the manufacture of printed circuit boards (PCBs), also known as Printed Wiring.
The circuit board is the dominant technology used in the assembly of discrete components into operating circuits. With improvements in electronic components and more stringent performance requirements for electronic circuits, the properties of the laminates used in printed circuit boards have also been upgraded, e.g. with fiberglass reinforced high performance plastics such as FR-4. While the use of those improved laminates made possible many advanced products, it also created numerous fabrication and performance problems. Strenuous physical requirements were imposed on laminates. The laminates must withstand violence during processing. They are hit and gouged with drill bits, sprayed with caustic, dipped in acid, soaked in solvents, blasted with heat during reflow, and finally subjected to very high temperatures in a solder wave bath.
Woven fiberglass reinforced plastic laminates such as epoxy, silicon, etc., are recognized as having the best mechanical, electrical and chemical properties as related to the requirements for printed circuit boards. These laminates have high bending strength, surface strength, volume resistivity and bonding strength. They also have low water absorption, low dielectric constant and a low dissipation factor. However, all of these desirable properties are characteristics of the laminate before it is fabricated into a printed circuit board, especially before the mounting holes are drilled and before the board is subjected to many chemical baths at different temperatures and subsequently stored in ambient atmosphere. The laminates are expected to survive printed circuit board fabrication essentially unchanged in mechanical and electrical properties and be able to withstand further miscellaneous abuse for an indefinite number of years. In practice, however, the mechanical as well as electrical properties of the circuit board are significantly reduced, and this has created numerous, persistent and well-recognized performance problems.
For example, in presently available laminates the reinforcing fibers are located practically at random throughout the laminate. It is often necessary to drill a large number of small holes in a printed circuit board, with each hole resulting in the breaking of many reinforcing fibers. It has been established by testing that discontinuous fibers or shorter fibers contribute less to the strength of a laminate than long continuous fibers. As described by A. Kelly and W. R. Tyson, High-Strength Material, published by John Wiley & Sons: "The average tensile stress in a fiber is always less than that found in continuous fibers. The strength of the composite is then always less than that found for continuous fibers. The strength increases as the length of the fibers increases." It can therefore be concluded that after drilling the board does not preserve the mechanical properties of the original laminate, since the fibers and surrounding laminate are broken up. Consequently, the board in its final form has a reduced mechanical strength.
Another factor that affects the mechanical strength of the laminate is the degree of adhesion of the plastic matrix to the fibers. It is customary to cover the fibers with an adhesion-promoting substance. However, during drilling there are fiber ends exposed on the inner surface of the hole, and fibers are pulled and twisted and substantially separated from surrounding plastic. This not only has an adverse effect on the mechanical properties of the laminate but also permits moisture to penetrate the plastic, e.g., when the laminate is later submerged in chemical baths. This creates current leakage paths between component leads and lowers the insulation properties of the circuit board, distorting the intended circuit performance. It also creates "blow-outs" during in-hole plating when this moisture hidden under the plating evaporates. These defects make the board useless, or nearly so, for many critical applications.
The volume resistivity is affected due to the voids created by hole drilling between the cut-off fiberglass ends and the plastic on the inside hole surface, and the resulting moisture entrance during bath processing. This will have an adverse effect on the generally low dielectric constant throughout the laminate, and the same is true with respect to the dissipation factor. All of these effects are in the most critical areas between closely spaced holes and consequently between component leads, thereby significantly degrading circuit performance.
Another well recognized drawback of the fiberglass reinforced laminate is that it is difficult to fabricate into a functional board, because the drilling or punching of holes is made very difficult due to the strength of the fibers. The have been many efforts improve the workability of a laminate.
One such effort is outlined in U.S. Pat. No. 4,550,051 to Sprelau et al. According to the teaching of this patent, the laminated is built of different outer and inner layers. The outer layers consist of fiberglass reinforced epoxy, while the inner core consists of resin impregnated flat textile forms of synthetic thermo-plastic fibers. The goal of this patent is apparently to construct a laminate that is easier to drill. However, quality is sacrificed by replacing fiberglass reinforcing with synthetic plastic fibers inside the laminate structure, and any improvement in workability is only partially achieved because some abrasive glass fibers still have to be cut in the drilling process.
The drilled holes also present a problem with respect to dimensional stability. The precise positioning of the holes is very important, but even if the holes are drilled precisely in the desired locations, the dimensions of the board change due to moisture absorption by the boards during processing steps in various baths as well by absorbing moisture from the air. The drilled holes with open internal surfaces with exposed broken glass fibers tend to increase this moisture absorption by the board. The large number of holes per board area substantially increases the moisture absorption and consequently the dimensional changes in the board.
Hole drilling has also presented a problem due to the abrasiveness of the glass fibers. Special hard alloys must be used for the drill bits, and the heat developed during drilling was found to soften the plastic and create smirs, necessitating the development of a technique for desmiring.
The above are only a few of the many serious performance as well as cost problems created by the design of the reinforced laminate.
A great deal of fine engineering has been devoted to the effort to inspect the boards at every step of production. Laminates are inspected before shipment and as received. All holes are inspected, after drilling, for hole surface quality using semi-automatic equipment. Assembled units are checked for solder faults, bridging, cold solder joints etc.
A rejection rate of 30 percent has been reported and is considered a necessary cost of doing business. The efforts have been directed to finding every fault on every board through inspection and to minimize the effect of some problems, rather than to find and eliminate the causes of those persistent problems.
A further drawback of present circuit boards relates to the size and spacing of the holes. The shape and the size of the mounting holes in the board are determined by the shape and size of the component leads as well by the fabricating process.
The holes in the fiber reinforced laminate circuit boards can be drilled, or sometimes punched. The drilled holes are round, by definition. The punched holes are also round, because a round punch is best able to withstand the punching pressure. On the other hand, the leads of the components to be mounted, e.g., a Dual In-Line Package (DIP), are rectangular. As shown in FIG. 5, the diameter DH of the mounting hole is therefore determined by the diagonal of the rectangular cross section of the lead, and the diameter is typically increased to compensate for dimensional tolerances of location. In FIG. 5, "l" and "n" represent the length and width, respectively, of the lead.
The round shape of the holes and flat shape of the leads creates numerous problems. To provide a solder connection, the hole is surrounded on the board by a conductive ring, or land, which is is connected to a conductor line and has a diameter DL. The holes are spaced at a distance "t"--referred to as the pitch. The distance C1 between the outside land diameters, considering the nominal dimensions as well as the dimensional tolerances, is often close and consequently results in solder bridging during wave soldering.
The close location of the lands also precludes the running of conductors between them, complicating the conductor patterns of the board and reducing the possible component density per square inch of board. When conductors are designed to run between solder lands, they are very narrow, and the fabrication of narrow conductors creates severe manufacturing problems.
A number of attempts have been made to improve the quality and manufacturability of printed circuit boards. U.S. Pat. No. 3,972,765 to Kondo et al addresses the difficulties in the drilling and punching of mounting holes in woven fiberglass reinforced epoxy impregnated laminates, with a goal of reducing the cost of the laminate and the cost of fabricating mounting holes. Kondo introduces a non-woven fabric of glass fiber impregnated with an epoxy resin as the prepreg, which may improve the workability as well as the fabricating cost of a laminate. However, Kondo does not address the problems created by the cutting of the glass fibers during the fabrication of holes.
U.S. Pat. No. 3,258,387 to Brown et al teaches a method of manufacturing laminates that provides a composite material in which mineral flakes are the major constituent and are oriented in parallel planes. Brown et al addresses the problem of the cost of laminate production, but does not address the effects of hole fabrication on the properties of a printed circuit board made from the laminate.
U.S. Pat. No. 3,616,613 to Benzinger et al addresses the workability of a laminate. Benzinger et al teaches a punchable printed circuit board laminate formed by laminating resin impregnated woven glass fiber sheets to the surface of an impregnated non-woven fiber glass core. The combination of woven and non-woven glass reinforced fibers might reduce the punching pressures to some extent. However, while the laminate is referred to as "punchable" it will require dies with strong punches. The cost of such dies can be justified only on long production runs of the same board. The problems of broken fibers are not addressed.
U.S. Pat. No. 3,988,408 to Haining et al teaches a process for stabilizing a printed circuit board substrate or laminate through application of a number of laminate conditioning and baking cycles which function to provide the laminates with dimensional stability. This addresses the problem of dimensional stability of the laminate, but does not address the problems created by the fabrication of mounting holes and the breaking of the reinforcing fibers.